Inside the solar-powered plane that crossed the US

Journey is intended to build interest in a round-the-world solar flight.

A view out to the end of the wing, which cants upward slightly. The wing supports a total of four engines, and is covered in solar panels.

John Timmer

Three years ago, as I was in Switzerland to visit the Large Hadron Collider, a cousin of mine mentioned that they were testing a solar-powered plane at the airfield near his home in Lausanne. If a lightweight, human-powered plane managed to cross the English Channel, a solar-powered craft seemed like an obvious extension–in fact, the same team later crossed the channel in a solar aircraft. Whatever the Swiss were up to, it didn't strike me as that big a step. But with the arrival of that same aircraft in New York City last week, I got a much better sense of what's going on.

In the intervening years, Solar Impulse proved that it WAS a bit more than some solar panels slapped onto a lightweight design. It managed to take off without any fuel and complete a flight that lasted more than 24 hours, and it later successfully flew from Europe to North Africa. Its arrival in New York City (see a gallery here) was the final leg in a flight that started over the Pacific Ocean near San Francisco. Ultimately, the aircraft is just a testbed for what will eventually be the first flight around the world to be performed without any fuel.

Solar Impulse, by the numbers

Despite its enormous wingspan (over 63 meters, or 208 feet), Solar Impulse is a relative lightweight, tipping the scales at 1,600kg (3,500lbs). Its basic shape is that of a glider: a long, extensive wing to provide lift and a narrow body to minimize wind resistance. In fact, it's part of the normal flight plan to use it as a glider for several hours (more on that below); nevertheless, it does have four engines, each rated at 10 horsepower, and these are capable of driving the propellers at 400rpm.

The huge wing also serves another purpose: hosting an enormous array of solar cells. Rather than using the most efficient technology out there, Solar Impulse relies on some of the thinnest and lightest-weight silicon photovoltaics. The particular cells in use on the plane come from SunPower and are only 135 microns thick. Between the wing and the tail, there are approximately 200 square meters of photovoltaic hardware, far more than is needed to drive the engines. That's because the excess is sent into batteries.

Even using lightweight lithium polymer material, the batteries account for about a quarter of the plane's mass. All told, they can store nearly 100kWhrs of power, and they're part of the key to the plane's 24-hour endurance. But only part. During the peak hours of sunlight, the aircraft is able to both charge its batteries and climb up to its peak altitude of about 9,000m (30,000 feet). Once the sun is no longer providing sufficient power, the pilots can just shut off the engines and glide for several hours before switching to battery power. With careful management, Solar Impulse will still be over 1000m above the ground by the time power starts coming up the next day. Using this technique, the team has set a record by staying aloft for 26 hours.

The present and future

In the same way that the aircraft is both big and small, the project as a whole both is and isn't about the future. Pilots Bertrand Piccard and André Borschberg made it clear that the project's intended message is that the sort of technology needed to get a solar-powered aircraft like Solar Impulse off the ground is a reality now. Everything that makes the plane work—the solar panels, the batteries, the engines, the lightweight polymers, and insulation—is being made and sold profitably, often by the companies that are now sponsoring Solar Impulse.

That doesn't necessarily mean they were ready-for-use in an aircraft like Solar Impulse. The initial work on the plane was done at the École Polytechnique Fédérale de Lausanne, with engineering work performed by the Solar Impulse team itself. Once that was done, many of the companies had to determine whether they offered, or could adapt, existing products to fit the design's specifications. A spokesman for the chemical giant, Solvay, for example, said his company undertook 50 research projects in response to the Solar Impulse design; about half worked out.

In any case, Piccard argues that these technologies are exactly what we need to make our existing standard of living sustainable. In his view, you can't adopt them in pieces; they have to be part of an integrated package, much like Solar Impulse itself. "To put solar power on a house with single glazed windows and no insulation is foolish" Piccard said at his New York press conference. "Make the house energy-efficient first, and then it can run on only solar power." To him, Solar Impulse is a flying advertisement for everything that efficient and renewable technology can already accomplish.

That said, the project is also meant to build interest for the next big step, a planned around-the-world flight tentatively being targeted for 2015. As Borschberg put it, the current plane is already capable of flying around the world. Previous flights have demonstrated its ability to sustain 24 hour flights. "The weak link is the pilot," Borschberg said. "The pilot is not sustainable yet." Right now, the pilot can grab some snacks while in the cockpit, but there's not a lot of room for anything else. Adding a second pilot probably isn't an option. Beyond the weight of the pilot, they'd need oxygen, food, water, a parachute, and more. Even in its current configuration, Borschberg estimated that the pilot adds about 225kg to the aircraft, which only weighs 1,600kg to start with.

So, the current plan is to build a larger aircraft, one that will allow the pilot to move around and sleep. They're still working on understanding whether it will make sense to carry oxygen and water, or if those can be efficiently recycled on-board. In any case, building a larger plane will clearly need some sponsors and backers, and part of the goal of the trip was undoubtedly to provide them with a demonstration of what they'd be backing.

This trip didn't go entirely smoothly, making it a learning experience for future efforts. The initial leg started from California's Moffett Field where Piccard took Solar Impulse out toward the Pacific for what he thought would be a simple photo-op flight over the Golden Gate Bridge. But the updrafts over the coastal hills were so intense that he ended up having to shut the engines off. Even then, he climbed well above his intended flight plan and had to ask the San Francisco Airport to reroute commercial traffic around him.

Borschberg ended up at the wheel when the landing in Dallas was complicated by headwinds that roughly matched the forward speed of the aircraft. The landing approach required that they face the aircraft into the wind and then drift sideways toward the airport before dropping low enough that the wind speed tailed off. He was also at the controls when some of the wing fabric tore off on the flight to Kennedy Airport in New York City. He took it philosophically, saying, "if I have to bail out, it'll be an interesting experience; if I land in JFK, it will be an interesting experience." Fortunately, the plane continued despite the tear, although it did have to land ahead of its planned arrival.

The project is focused on solar-powered flight, but the team hasn't been focused on promoting that as a future anybody will enjoy. It makes sense, given that the Sun doesn't provide light in a concentrated enough form to ever really propel anything substantial at a decent speed. But the project does make a compelling case: efficient, renewable technology has reached an impressive level of maturity. A maturity that just might take it around the world.